Immunohistochemical studies have revealed that galanin (GAL) and vasopressin (VP) coexist in the very same cell bodies in the supraoptic and magnocellular paraventricular nuclei and the accessory cells of the lateral hypothalamic area. Dehydration and salt loading, which is known to cause release and depletion of VP and oxytocin from the neurohypophysis, also caused a marked reduction of GAL-like immunoreactivity in the posterior lobe of the pituitary. Effects of systemically administered GAL caused a short lasting small increase in blood pressure with no effect on heart rate. A thousand-fold molar concentration of GAL, compared to VP, was required to cause comparable effects on blood pressure. GAL had no modulatory effect on VP-induced response. Systemically administered GAL resulted in a mild increase in diuresis whereas VP caused complete and sustained inhibition of diuresis. GAL had no effect on VP-induced antidiuretic effects. The significance of the coexistence of GAL and VP remains to be elucidated.